<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* 0, 0, 0, 1 */
    .box {
      width: 200px;
      height: 200px;
      background-color: pink;
    }
    /* 0, 1, 0, 0 */
    #first {
      border: 1px solid blue;
    }
    /* 层叠性 */
    #first {
      border: 1px solid green;
    }
    .box#first {
      border: 1px solid red;
    }
  </style>
</head>
<body>
  <div class="box"></div>
  <div class="box" id="first"></div>
</body>
</html>